Dwindling budgets and the uncertainty about the Highway Trust Fund have caused departments of transportation and municipalities to take longer looks at pavement preservation techniques.
In light of this, more transportation entities are looking at thin overlays—commonly called thinlays—as a way to rehabilitate roads rather than replace them.
Asphalt thinlays, which range from 3/4-inch to 1-inch thick, are created with smaller nominal maximum aggregate sizes than typical asphalt pavement and are supplemented with more asphalt binder mix that is formulated to be softer. That helps make the mix more durable and resistant to cracking—ideal attributes for a preservation product.
Chuck Fuller, president of J.D. Ramming Paving in Austin, Texas, has been working with thinlays for more than eight years. He’s also involved with the National Asphalt Pavement Association in their pavement preservation educational efforts, and has spoken about his experience with thinlays at an education session at the World of Asphalt show.
While thinlay use is growing, Fuller says, there also is some trepidation because of some cases of misuse, cases that lead to mixed results and even failures.
“Thinlays are a pavement preservation tool,” he says. “That needs to be stressed more than anything. A thinlay is not a dense-graded material. It does have structural value, but it’s not to be used solely for a typical structural valued roadway.” Fuller’s company learned some valuable lessons on a project that used thinlay instead of a standard-thickness mat to widen a roadway to create turning lanes. The pavement quickly failed.
The underlying structure of the roadbed, he explains, has to be sound for a thinlay to perform properly. For example, if there is block cracking, then a contractor has to crack seal the roadway or apply an underseal membrane on the material.
But because thinlays have had a high success rate in preserving roadways – and generally are thought to extend the life of a pavement for at least seven years – there is a rush to use this method outside of its intended application, Fuller says.
“It’s an ongoing battle for proper use, especially with some of the entities out there, because they see the value in it,” Fuller says. “It costs more per ton to produce and place, but a ton of it goes a long way, especially at less than an inch.”
As a result, he adds, some agencies want to use it in all situations that call for a pavement preservation method. “We have conversations many times with our clients about a certain pavement condition not being the correct application for this product, and that the situation needs to be reevaluated,” Fuller says.
“We may go out and do some of the maintenance work that needs to be done under the thinlay, and then it (thinlay work) is still not produced,” he adds, explaining that more maintenance work beyond the requirements of a thinlay are needed in these instances.
Fuller says his company has performed thinlay work that continues to perform well and look good after eight seasons, and that he estimates could last 10 to 12 years without maintenance. But, he admits, there have been failures.
“I’ve had thinlays fail within one season when they’re used in the wrong application, or they’re used without doing the correct preparation work,” he adds. “We’ve experienced both the good and the bad.”
John Hickey, executive director of the Asphalt Pavement Association of Oregon, says the biggest factor contributing to the growth of thinlay application is the increasing need for effective pavement preservation techniques.
“DOTs have lived through the seals, and now they’ve gotten to the point that doing them is not a long-term solution,” he says. “Dealing with preservation is the biggest challenge for pavement managers. They just have more preservation work to do.”
Hickey says the additional challenges of aging roadways coupled with long-term budget reductions have taken their toll on pavement managers, causing them to seek better preservation solutions. “The general feeling in the pavement management community is that seal treatments are not providing the ideal benefits,” he says. “They don’t extend structural life.”
Because DOTs have been so focused on creating long-life pavements, Hickey says his group has been promoting how thinlays can achieve this goal.
“We tout the benefit that you can get to a perpetual pavement 1 inch at a time,” he says. “With our growing preservation needs and people looking at more life cycle cost concepts and preservation, we’re moving away from ‘Oh, I’ve got to do some sort of preservation, let me just throw some money at it.’”
But like Fuller, Hickey stresses that thinlay application isn’t a silver bullet, and contractors cannot apply a thinlay on a structurally failed road and expect it to endure.
And other times thinlay application mistakes may be due to confusion over the difference between a thinlay and a thinlift. “We can’t just equate thinlift with thinlay,” Hickey says.
Thinlifts are standard mix designs placed thin; thinlays have a completely different mix design. “Even in the asphalt community there’s some confusion, and it’s a mistake I regularly see. From a design and engineering standpoint, it’s different.” Hickey references the specific differences of smaller aggregate size and the softer binder grade. Softer binders are more crack resistant, whereas stiffer binders, which are used in thinlifts, are more susceptible to cracking.
The condition of a pavement is the first consideration when deciding to use a thinlay (see sidebar), but the main way to ensure long-term success is site preparation, which needs to be carefully planned and executed.
Fuller says one of the biggest factors is cleanliness, particularly if any milling is involved. “We have found that the lack of preparation of the underlying surface – patching, crack sealing and cleanliness – is the number one reason for premature failure,” he says.
He adds that his company primarily uses brooms to clean prior to application, in addition to minimal use of vacuum trucks. On one section of roadway, they even used a water truck with a broom to clean. “We found that with the slightest amount dust, especially on trackless tack, the mat does not bond and slippage occurs,” he says.
“Cleaning of the roadway before you put out a tack coat is imperative,” Fuller says. “And applying too much tack coat can cause other problems. I’ve seen several pavements fail with the tack material coming up through the thinlay, and then the wheel paths will have what looks like bleeding or seeping.”
Fuller has found when milling it’s been effective to allow traffic to run on the milled surface a day before thinlay application. “Traffic helps remove the dust,” he says, “and it ensures that the surface is dry.”
The smaller nominal aggregate size used in thinlays presents a transport challenge for the mix, as it has a tendency to lose heat quickly. Fuller says insulated trucks are a must when hauling the mix, and also advises placing tarps over the mix, particularly with distances of more than 25 miles from plant to jobsite.
“When you start dealing with thinlay mixes and manipulating them, they can start to chunk,” he says. “So when you’re trying to lay a 1-inch mat, and these cold chunks come through, they get caught underneath the screed and there’s nothing you can do besides stop, pick the screed up, and then start again.”
This starting and stopping with a thinlay mix is problematic because Fuller says it can take roughly 25 feet for the screed to level itself out, causing a shadowing effect.
“With a thinlay it’s real important that once you start, you don’t stop,” he adds. “It’s more visual than anything, because I’m not seeing where there are premature failures in those areas.”
These examples aside, Fuller’s main point about application is simply to follow basic standard paving procedures. “Essentially you’ve got to go back to ‘Paving 101.’ Everything you do wrong is magnified drastically with a thinlay.”
Pavement managers are trying to think ahead and address the concerns of the traveling public, Hickey says, in addition to using pavement preservation techniques that last and provide value.
Unlike seals, thinlays also create a smooth roadway, a feature that gets the most attention from motorists.
“The number one thing the traveling public cares about is smoothness – a nice, smooth, quiet ride,” Hickey says. “A chip seal doesn’t give you that; it gives you a bumpy noisy ride. Putting down a thinlay gives motorists the impression of a new pavement.”
Pavement managers, Hickey says, are starting to look at preservation from a life cycle cost perspective and the best use or resources. “The idea of actually putting pavement down on the surface where it’s going to make it impermeable, improve smoothness, and add structure – it’s very enticing, and I think we’re going to see continued growth in thinlays because of that,” Hickey concludes.